Native Son(1986) [Mary Dalton]: Accidentally smothered by Victor Love when he puts a pillow over her face to keep her quiet when the blind Carroll Baker enters Elizabeth's bedroom; he discovers that she's dead after Carroll leaves. He then takes her body downstairs and places her in the furnace to dispose of the body. (See also Jean Wallace in the 1951 version.) (Thanks to Drew)

The Changeling(1994) [Beatrice-Joanna]: Stabbed by Bob Hoskins; she dies shortly after confessing her part in his crimes. (I haven't seen this version, but I'm familiar with the play.)

Poirot: Appointment with Death(2008) [Dame Celia Westholme]: Poisoned with an injection of digitalis by John Hannah. (Thanks to Brian)

Clash of the Titans(2010) [Marmara]: Drowned, along with Sinead Michael and Pete Postlethwaite, when Ralph Fiennes uses his godly powers to sink their boat.

Kick-Ass(2010) [Mrs. Lizewski]: Dies of an aneurysm while having breakfast with Aaron Johnson and Garrett M. Brown. (Played for comic effect.) (Thanks to Tommy and David)